Man-wai Chang to The Door (33600bps) said:
http://www.uxcell.com/way-silvery-catv-cable-signal-splitter-combiner-p-37221.html
I noticed an electronics shop selling this kind of TV splitter at
different impedance (or is it resistance?). Should I just go for the
highest impedance for DMB-TH + PAL signal?
http://www.epanorama.net/links/wire_av.html
"The RF signal is generally split to multiple TV antenna outlets using
splitters. The loss of a 2-way splitter is approximately 4-6dB and
6-9dB for a 4-way type. Signal loss should be compensated by an
amplifying signal with suitable amplifier. Splitters are used to
split one incoming wire to two or more outputs. Splitters split the
incoming signal power to the outputs evenly and maintain the
impedance matching on the systems. Splitters provide isolation between
receivers (Without isolation one receiver could degrade the quality of
the signals going to other receivers). When using splitters be sure that
all unused outputs are terminated with 75 ohm load or terminator to
avoid signal reflections and degration of thes plitter performance."
The connectors on your splitter, are F-series intended for 75 ohm operation.
The cable TV wire is 75 ohm. The splitter should be 75 ohm as well. All
components in a "transmission line system" must match impedance, in order
for there to be no reflections.
Not all of the properties in Tomi's description above, are necessarily
true. The receivers connected to the splitter, may not be isolated
from one another (there are resistive splitters and transformer based
splitters). That is why all the wiring and terminations must match,
so that what you do to one port on the splitter, does not affect the other
port. As long as the ports have the correct impedance connected to them,
then it doesn't matter what kind of splitter is used.
If I was buying a splitter, I would only buy "enough ports" for what I was
doing. For example, I have a TV antenna, 20dB amp, and long run of cable
leading to my computer room. (Doing it this way, prevents the TV antenna
from picking up RF noise from the computer.) In the computer room, I have
a 2 way, 75 ohm splitter. One port connects to a TV set, the other to a VCR.
I have used all the ports on the splitter, so I don't have to shop for
terminators or have to make my own. There are no open connectors on
the splitter that I have to worry about. (An open coax cable, with
no load on the end, is *not* terminated. Either you have a TV on the
end of the cable, or a terminator plug, to complete the circuit.)
TV (75 ohms input)
2 way /
Antenna --- 20dB amp --- 75 ohm cable --- splitter
\
VCR (75 ohm input)
If you buy a 4 port splitter, and only use 3 ports, you should place a
terminator on the remaining port. Those are not easy to find. You can
make your own, but it would likely not perform very well. I've *never*
seen one of these at retail electronics stores or in radio/TV stores.
http://www.hollandelectronics.com/catalog/catalog.php?product_id=F-Series-Terminators
By capping the unused port with a terminator, that stops signal leakage.
That might be appreciated by your neighbors who are receiving OTA
(over the air) TV signals, while you are enjoying your cable TV.
Capping the port, also meets the matching requirements for coaxial
cables.
So when I bought my splitter, there were 3 or 4 port ones for sale, but
I knew exactly how many ports I needed, and all ports are connected
to an electrical load.
With respect to the comment about amplification, that isn't always necessary.
TV sets have an amazing RF input range, capable of handling a 5 to 6
orders of magnitude difference in signal level. A little loss caused by a
splitter, is of no consequence. As far as I know, cable TV signals aren't
super strong, they're roughly in the middle of the range of the TV set (1 millivolt
or so ?). The splitter should not really degrade the signal to the point that
it becomes snowy or noisy. An extremely long length of RF cable could be
an issue, as RF cable has loss (so many dB per thousand feet), and if
the run is long enough, eventually the TV picture would become snowy.
If you wish more isolation between ports (such that no matter what happens
on one port, it doesn't affect the others), you should be using a
distribution amplifier. Those cost more, require a power source, but
ensure that even if one of the ports has a dead short on it,
the other ports would continue to work. Amplifiers are not always the
best thing to use, because a broadband amplifier can amplify non-TV noise
as well as TV signal. With sufficient amplification, things like taxi radio
channels, can start to interfere with the TV set. So the usage of infinite
amounts of RF amplification is not recommended. When I mentioned that I have a
20dB amp on my OTA TV antenna, that helps with fringe reception and increases
the number of stations I get. (I'm a cheap bastard, and I will not pay for
cable...)
So in answer to your question, if the TV has a 75 ohm input, the cable TV
company is using 75 ohm cables, you should be using a 75 ohm splitter.
Or, if you have money to waste, you could also buy a 75 ohm distribution
amplifier. But that would normally not be necessary.
I've never taken one apart, to look inside. I found a patent for a very
complicated splitter, but I doubt this is what ships in most
radio/TV stores. The ones we get are likely much simpler designs.
http://www.freepatentsonline.com/6578202.html
Paul
